linux-kernel - Re: [PATCH v2 10/29] arm_mpam: Add cpuhp callbacks to probe MSC hardware

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Message-ID: <59fbb8d9-5df6-4269-aa81-147618450644@arm.com>
Date: Mon, 29 Sep 2025 18:44:16 +0100
From: James Morse <james.morse@....com>
To: Ben Horgan <ben.horgan@....com>, linux-kernel@...r.kernel.org,
 linux-arm-kernel@...ts.infradead.org, linux-acpi@...r.kernel.org
Cc: D Scott Phillips OS <scott@...amperecomputing.com>,
 carl@...amperecomputing.com, lcherian@...vell.com,
 bobo.shaobowang@...wei.com, tan.shaopeng@...itsu.com,
 baolin.wang@...ux.alibaba.com, Jamie Iles <quic_jiles@...cinc.com>,
 Xin Hao <xhao@...ux.alibaba.com>, peternewman@...gle.com,
 dfustini@...libre.com, amitsinght@...vell.com,
 David Hildenbrand <david@...hat.com>, Dave Martin <dave.martin@....com>,
 Koba Ko <kobak@...dia.com>, Shanker Donthineni <sdonthineni@...dia.com>,
 fenghuay@...dia.com, baisheng.gao@...soc.com,
 Jonathan Cameron <jonathan.cameron@...wei.com>, Rob Herring
 <robh@...nel.org>, Rohit Mathew <rohit.mathew@....com>,
 Rafael Wysocki <rafael@...nel.org>, Len Brown <lenb@...nel.org>,
 Lorenzo Pieralisi <lpieralisi@...nel.org>, Hanjun Guo
 <guohanjun@...wei.com>, Sudeep Holla <sudeep.holla@....com>,
 Catalin Marinas <catalin.marinas@....com>, Will Deacon <will@...nel.org>,
 Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
 Danilo Krummrich <dakr@...nel.org>, Lecopzer Chen <lecopzerc@...dia.com>
Subject: Re: [PATCH v2 10/29] arm_mpam: Add cpuhp callbacks to probe MSC
 hardware

Hi Ben,

On 12/09/2025 11:42, Ben Horgan wrote:
> On 9/10/25 21:42, James Morse wrote:
>> Because an MSC can only by accessed from the CPUs in its cpu-affinity
>> set we need to be running on one of those CPUs to probe the MSC
>> hardware.
>>
>> Do this work in the cpuhp callback. Probing the hardware will only
>> happen before MPAM is enabled, walk all the MSCs and probe those we can
>> reach that haven't already been probed as each CPU's online call is made.
>>
>> This adds the low-level MSC register accessors.
>>
>> Once all MSCs reported by the firmware have been probed from a CPU in
>> their respective cpu-affinity set, the probe-time cpuhp callbacks are
>> replaced.  The replacement callbacks will ultimately need to handle
>> save/restore of the runtime MSC state across power transitions, but for
>> now there is nothing to do in them: so do nothing.
>>
>> The architecture's context switch code will be enabled by a static-key,
>> this can be set by mpam_enable(), but must be done from process context,
>> not a cpuhp callback because both take the cpuhp lock.
>> Whenever a new MSC has been probed, the mpam_enable() work is scheduled
>> to test if all the MSCs have been probed. If probing fails, mpam_disable()
>> is scheduled to unregister the cpuhp callbacks and free memory.
>>
>> CC: Lecopzer Chen <lecopzerc@...dia.com>
>> Signed-off-by: James Morse <james.morse@....com>
>> ---
>> Changes since v1:
>>  * Removed register bounds check. If the firmware tables are wrong the
>>    resulting translation fault should be enough to debug this.
>>  * Removed '&' in front of a function pointer.
>>  * Pulled mpam_disable() into this patch.
>>  * Disable mpam when probing fails to avoid extra work on broken platforms.
>>  * Added mpam_disbale_reason as there are now two non-debug reasons for this
>>    to happen.
> 
> Looks good to me.
> 
> Reviewed-by: Ben Horgan <ben.horgan@....com>


Thanks!

James